A disconnector is a
mechanism built into
semi automatic firearms to make sure it only fires in
semi automatic mode, without the disconnector an autoloader would fire in fully automatic mode.
What the disconnector does is, as its name implies, disconnects the trigger mechanism from tripping the sear (on hammer fired guns) or releasing the striker (on striker fired guns) until the trigger is released and allowed to travel forward enough to reset.
This allows the pistol to only fire one round for every time the trigger is pulled until all ammunition is expended.
Mucking around with the disconnector or with the sear/striker engagement can cause a semi auto gun to go full auto without warning.